The present invention relates to digitally controlled variable impedance circuits and specifically to an analog output circuit which simulates a two-wire rheostat whose value is digitally controlled.
In the past, a number of circuits having a digitally controlled variable impedance have been developed. The extent of these prior art devices is given by the following patents:
U.S. Pat. No. 3,947,701 issued to Russell, Jr. on Mar. 30, 1976; U.S. Pat. No. 4,220,875 issued to Lawton on Sept. 2, 1980; U.S. Pat. No. 4,276,604 issued to Kitamura et al on June 30, 1981; U.S. Pat. No. 4,338,531 issued to Corso on July 6, 1982; and U.S. Pat. No. 4,408,514 issued to Suzuki on Oct. 11, 1983.
Each of the prior art devices requires an elaborate control circuit to translate the digital commands for the variable impedance circuit. Russell, Jr. discloses a digitally controlled variable impedance including a variable resistance connected to a counting register. The register is set to a predetermined count and operates to change the impedance value in response to receipt of each pulse of a pulse train. In Kitamura et al a digitally operated control circuit controls the attenuation factor of a variable loss attenuation circuit. The control circuit, which may be a microcomputer, is so programmed that the attenuation factor of the attenuation circuit is stepwisely increased. Suzuki shows a keyboard controlled circuit for a musical instrument including an amplifier. Lawton discloses a circuit having its impedance controlled by an external signal and which includes a feedback loop connecting the output of the second stage to the input of the first stage. Corso is directed to a duty cycle circuit for simulating a slide wire device.
Most of the above prior art devices employ bulky or relatively large components due to their elaborate control circuits which range from the microcomputer of Kitamura to a manually operated keyboard of the Suzuki device. In many applications, a digitally controlled variable resistor is required that is a single completely solid state circuit, including the control circuit.
In view of the foregoing discussion, it is apparent that there currently exists the need for an improved digitally controlled variable resistance circuit which, including the control curcuit, is a completely solid-state device. The present invention is directed towards satisfying that need.